The presence of sub-micron high aspect ratio features on Very Large Scale Integration (VLSI) semiconductor devices gives rise to the need to coat contacts on the bottoms of such features or to fill such features with conductive metals in the course of semiconductor device manufacture. In many semiconductor device manufacturing applications, it is required or at least preferred to apply the coatings using physical vapor deposition processes. This need or preference has resulted in a need for directing sputtered materials toward the substrate in straight lines oriented normal to the substrate surface.
For example, in semiconductor device manufacture, it is necessary to metallize contacts at the bottoms of high aspect ratio lines and vias that are in the range of 0.25 to 0.35 microns in width. Metallizing such contacts by a sputter coating processes is desirable because sputtering presents commercial advantages in time, cost and simplicity of equipment over alternative processes, particularly where devices on the substrate would sustain damage if subjected to the elevated temperatures required with coating processes such as chemical vapor deposition (CVD). With the decreasing size of features, high aspect ratios of features and the preferability of applying coatings by physical vapor deposition in certain applications, increasing demands are made on the sputtering process to achieve higher and higher degrees of directionality of the sputtered material. Unless the paths of the particles of sputtered material incident onto the substrate can be maintained highly parallel and normal to the substrate surface, the attempt to sputter coat the bottoms of the high aspect ratio holes will result in coating the tops and sides of the holes, in which event the sputter coating process does not achieve results that are satisfactory.
A sputter coating process is typically carried out by placing the substrate and a target of coating material into a vacuum chamber filled with an inert gas such as argon and creating a plasma in the gas, with the target being maintained at a negative potential, functioning as a cathode which supplies electrons to sustain the plasma. The target is typically part of a magnetron cathode assembly in which magnets behind the target trap the electrons over the surface of the target where they collide with atoms of the argon gas, stripping electrons from the atoms and converting them into positive ions. The argon ions are accelerated toward the negatively charged target where they collide with the surface and eject particles of target material. The ejected particles of target material propagate through the vacuum space where some strike and coat the substrate.
Various proposals have been made for causing the propagating particles to move in straight lines toward and normal to the substrate surface. Interposing a collimator between the target and substrate is one such method of achieving normal angles of incidence and improving incident particle directionality. Increasing the target to substrate spacing, known as long-throw sputtering, is another. Collimators provide a source of particulate contamination while both of these methods tend to substantially decrease the deposition rate.
A further method of directing sputtered material that has been given renewed consideration is the process of ionized sputtering. With ionized sputtering, often referred to as Ionized Physical Vapor Deposition or IPVD, coating material is sputtered from a target using conventional magnetron sputtering techniques, with a target energized with a negative DC or pulsed DC potential to release electrons, which produce positive ions of the gas in the chamber, which are attracted toward the target where they strike the surface and dislodge particles of the coating material. In IPVD, an additional plasma is created, such as by reactively coupling RF energy into the chamber downstream of the target, to ionize the sputtered material. A negative bias applied to the substrate attracts the positively ionized material particles, electrically accelerating them toward the substrate.
Research with IPVD has revealed that IPVD processes possess a number of drawbacks and problems that have precluded their practical use. Such processes have, for example, produced low overall efficiency. In particular, IPVD processes typically yield low deposition rates, with low ionization of the sputtered material and high film contamination. For example, with IPVD proposals of the prior art, the filling of high aspect ratio features has been found to deteriorate as sputtering power at the target increases. Such deterioration has limited the sputtering of aluminum alloys to 0.3 to 3 kW of DC power with a 12 inch magnetron target as compared to 12 to 30 kW that is typical for such target. The low sputtering power results in a low deposition rate that yields low productivity, e.g., 10 to 40 minutes of sputtering time per wafer as compared to the typical 45 seconds to 1 minute. Further, the coupling efficiency of RF energy into sputtered material has been found to be low unless operated at high pressure in the sputtering chamber, such as 30 to 40 mTorr of argon process gas as compared to sputtering pressures that are typically in the 1 to 5 mTorr range. The higher pressure results in poorer film properties and greater chamber and film contamination. Other problems that have resulted with IPVD are the sputtering of the RF electrode or element by the plasmas, the flaking of accumulated sputtered material that has deposited on the RF element, the shorting of the RF element by the plasmas or material that deposits on the element, and other plasma and material interactions with the electrode or element used to couple the RF energy into the plasma to ionize the sputtered material.
Accordingly, there is a need for an IPVD apparatus and method thereof that overcomes the drawbacks and problems of the prior art. In particular, there is a need for a practical and effective IPVD apparatus and method that produces acceptably high overall efficiency, particularly high deposition rates, high sputtered material ionization efficiency and low contamination of the deposited film.